1. Field of the Invention
The present invention relates to a process of poly-crystallizing amorphous semiconductor layers using laser annealing.
2. Description of the Related Art
Thin-film transistors (hereinafter referred to as TFTs), in which polycrystalline silicon (p-Si) is used as an active layer, have a higher switching capability than do TFTs in which amorphous silicon (a-Si) is used. Moreover, thin-film transistors are characterized in that, because the channel position in an active layer is determined through self alignment, elements can be miniaturized and converted into a CMOS (complementary MOS) structure. For that reason, thin-film transistors are used as pixel switching elements and drivers for active-matrix-type flat panel displays, e.g. liquid crystal displays.
A polycrystalline silicon film can be formed through heating and poly-crystallizing an amorphous silicon film. However, a low-melting point glass substrate is often used as a silicon film forming substrate because an inexpensive and large-sized substrate can be easily fabricated. In addition to the solid phase growing method, the rapid-thermal-lamp annealing (RTA) and the laser annealing have been proposed as poly-crystallizing methods. For a low-melting point of the glass substrate in particular, a low-temperature processable laser annealing is considered effective for poly-crystallizing annealing.
In the laser annealing shown in FIG. 1, an object 200 to be processed is placed in the annealing chamber 100 and an external light source irradiates focused laser light (Excimer laser) onto the object through the window (process window) 120. A nitrogen atmosphere at a normal pressure used as an annealing process atmosphere can provide high throughput and good productivity in the annealing process. Therefore, the poly-crystallization laser annealing in a nitrogen atmosphere under normal pressure has been employed to mass produce polycrystalline silicon layers.
The process atmosphere in the chamber during annealing is one important factor influencing the surface roughness and crystallizability of a p-Si. Excimer laser annealing in a nitrogen atmosphere under normal pressure can realize a high productivity of a polycrystalline silicon film. However, research of the present applicant demonstrated that it is difficult to control the surface roughness of a p-Si manufactured in the nitrogen atmosphere under the normal pressure, and that under such conditions the Si surface tends to be rough. In order to prevent formation of rough surfaces, annealing is continued while the chamber 100 is evacuated using the extraction pump 300 as shown in FIG. 1.
However, it has been revealed that performing Excimer laser annealing in the vacuum atmosphere leads to contaminating inside of the process window of the annealing chamber. Because the contamination rate is very swift, the cycle between contamination removal is very short. As a result, it has been difficult to employ the above-mentioned vacuum conditions in mass production poly-crystallization laser annealing apparatuses.
The present invention is made to overcome the above-described problems. It is an object of the present invention to provide a laser annealing method of forming a polycrystalline semiconductor film with smooth surfaces. This method is suitable for mass production, with less contamination of the process window.
In order to accomplish this object, a method for manufacturing a polycrystalline semiconductor device comprises a step of laser annealing an amorphous semiconductor layer in a low degree vacuum atmosphere.
In another aspect of the present invention, a method of manufacturing a thin-film transistor comprises the steps of forming an amorphous silicon layer on a substrate; disposing the substrate on an annealing chamber; creating a low degree vacuum atmosphere within said annealing chamber; and irradiating focused laser light onto the amorphous silicon layer overlying the substrate through a chamber window built in the annealing chamber to anneal and poly-crystallize the amorphous silicon, whereby a polycrystalline silicon layer is formed as an active layer of the thin-film transistor.
In another aspect of the present invention, the annealing is performed under a pressure between about 1.3xc3x97103 Pa and about 1.3 Pa.
In another aspect of the present invention, the annealing is performed in an annealing atmosphere containing an inert gas.
As described above, the polycrystalline semiconductor layer, e.g. polycrystalline silicon layer, formed through the laser annealing in a nitrogen atmosphere under a normal pressure (atmospheric pressure) is relatively very rough. The following is considered as one of causes. It can be considered that this reduction in flatness is caused by formation of nitrogen reaction film on the surface of the amorphous semiconductor layer due to the presence of nitrogen in the annealing atmosphere. In contrast, laser annealing under high degree vacuum condition does not lead to formation of reacted products on the film surface, and a polycrystalline semiconductor layer with a smooth surface can be formed. However, the vacuum created inside the annealing chamber causes an increase in the mean free path of gas molecules, with the result that, during the laser annealing, particles sputtered around a semiconductor layer placed in the chamber will travel to and deposit onto the chamber window, thus contaminating it.
In the present invention, by conducting laser annealing under a relatively low pressure, for example, ranging from about 1.3xc3x97103 Pa to about 1.3 Pa, formation of reactive products on the surface of an amorphous semiconductor layer can be prevented. As a result, a polycrystalline semiconductor layer having high surface smoothness can be manufactured. Moreover, since the mean free path of molecules can be reduced compared with vacuum (high degree vacuum) conditions, the contamination rate of the chamber window can be sufficiently reduced. As a result, a polycrystalline semiconductor layer with uniform grain diameter and with flat surface can be manufactured with high productivity. The polycrystalline silicon layer thus obtained can be used as an active layer of a thin-film transistor. For example, in the top gate-type thin film transistor, the gate electrode can be formed above the polycrystalline silicon active film with high smoothness. The electrical and structural process margin can be sufficiently secured. Thus, transistors with high reliability can be manufactured.
In another aspect of the present invention, a laser annealing apparatus, wherein focused laser light is irradiated through a chamber window onto an object to be processed placed inside an annealing chamber, comprises an introducer for introducing an inert gas into the annealing chamber during annealing; a pump for reducing the pressure in the annealing chamber; and a pressure controller for controlling the pressure in said annealing chamber to a pressure between about 1.3xc3x97103 Pa and about 1.3 Pa.
In the above-mentioned apparatus structure, a polycrystalline semiconductor layer with high surface smoothness can be effectively manufactured in the annealing chamber.
As described above, according to the present invention, stable polycrystalline semiconductor layers with high smoothness can be manufactured with high productivity through low degree vacuum laser annealing.